In recent years, color liquid crystal display devices have been broadly used as monitors for portable notebook-size personal computers, personal computers and the like, or as display units for liquid crystal television sets, video integrated liquid crystal television sets, cellular phones, portable information terminals and the like in various fields. With an increase of an information processing amount, diversification of needs, adaptation to multimedia and the like, enlargement of a screen, and enhancement of definition of the liquid crystal display device have been vigorously advanced.
The liquid crystal display device basically comprises a backlight unit and a liquid crystal display element unit. As the backlight unit, there is an under light system in which a light source is disposed right under the liquid crystal display element unit or an edge light system in which a light source is disposed facing a side face of a light guide. From a viewpoint of a compact liquid crystal display device, the edge light system has been frequently used.
Additionally, since a polarization direction of light emitted from the backlight unit has heretofore been uneven, about half of the light has Seen absorbed and wasted by a polarization plate disposed on an incidence side of the liquid crystal display unit.
As a solution of this problem, as described in JP(A)-9-506984, a method is used in which a polarization separation element having a function of transmitting one polarization component of the light and reflecting the other polarization component is disposed on an emitting face of the backlight unit. A polarization transmission direction of the polarization plate disposed on the incidence side of the liquid crystal display element is matched with that of the polarization separation element. Accordingly, the polarization component which has heretofore been absorbed by the polarization plate and wasted is reflected by the polarization separation element, and returned to the backlight unit. The light returned to the backlight unit is reflected by the back surface of the light guide or the like, and strikes on the polarization separation element again. The light returned to the backlight unit changes its polarized state while reflection is repeated in the light guide. A part of the light which has entered the polarization separation element passes through the polarization separation element, and the other light is reflected again. In this manner, if there is not any loss of quantity of light while the light reciprocates between the polarization separation element and the backlight unit, all the light eventually passes through the polarization separation element. There list not be any light that has heretofore been absorbed by the polarization plate of the liquid crystal display element unit.
However, in actuality, since there occurs a loss of quantity of light every time the light reflected by the polarization separation element is reflected by the backlight unit, luminance is not enhanced according to the theory. Then, in order to enhance a ratio at which the light reflected by the backlight unit strikes on the polarization separation element again, it has been proposed in JP(A)-11-352479 that a regular reflection sheet be used on the back surface of the light guide. To further enlarge a change of the polarized state by the reflection in the backlight unit, it has been described in JP(A)-11-142849 that elongated prisms be disposed on the back surface of the light guide, and formed in such a manner as to extend in an oblique direction with respect to a polarization transmission plane of the polarization separation element.
However, even if these methods are used, the loss by the reflection between the polarization separation element and the backlight unit cannot be sufficiently reduced, and an effect of enhancing the luminance by the use of the polarization separation element is not sufficient.